17773( 3) - 10-Mar-2025 14:09:12 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17773 Version: 3 Check-in Time: 10-Mar-2025 14:09:12 STScI Edit Number: 0 Title Searching for Once-Oceanic Salts on Charon with HST/STIS ------------------------------------------------------------------------------------ Type Cycle GO 32 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Samantha Trumbo University of California - San Diego Y CoI: Dr. Michael E Brown California Institute of Technology N ------------------------------------------------------------------------------------ Abstract The New Horizons mission revealed that Charon could be an example of an "extinct ocean world", whose once liquid layer has completely frozen. The extensive tectonism and smooth plains on Charon are consistent with the pressure-driven resurfacing of a freezing subsurface ocean, and suggest that once-oceanic materials could have reached the surface. Intriguingly, Charon's infrared spectrum shows evidence for widespread ammoniated material suggested to be possible ammonia hydrates or, most recently, ammonium salts like NH4Cl, both of which could have become concentrated as the subsurface liquid froze. While this interpretation from just the infrared data is ambiguous, visible-wavelength spectra sensitive to absorptions from irradiation-induced crystal defects known as "color centers" provide a diagnostic means of salt detection that has been successfully implemented for Europa, Ceres, and Mars. Both the proposed NH4Cl on Charon and the NaCl detected on these other bodies should form during the freezing of NH4-, Cl-, and Na-bearing brines and, thus, are plausible surface materials for Charon. Both salts exhibit strong color-center absorptions across the 300 - 550 nm wavelengths that have remained totally unexplored for Charon in the nearly 50 years since its discovery. We propose a simple, single-orbit program with HST/STIS that will provide the first-ever spectral coverage of Charon across these wavelengths and enable a robust search for these salt color-center absorptions. This single-orbit request has the potential to transform our understanding of the connection between Charon's long-changed internal structure and its modern-day surface. ------------------------------------------------------------------------------------ Observations Description ------------------------ The observations entail a single orbit of HST, druring which we will acquire and observe Charon with STIS using the 52"x0.2" slit and G430L first-order spectroscopy mode. Our observations are straightforward and consist of a single visit and pointing, including one science exposure with the maximum exposure time allowable by the orbit constraints. To maximize this exposure time, we defer the wavelength calibration until after Earth occultation. ------------------------------------------------------------------------------------ 17773( 3) - 10-Mar-2025 14:09:12 - [ 2] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 TARGET NAME: CHARON ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE Charon ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 STD=PLUTO STD=CHARON ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 16.3 +/- 0.1 Ephemeris Center: EARTH Extended: NO ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 2 TARGET NAME: PLUTO ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: DWARF-PLANET Pluto ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 STD=PLUTO ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 14.55 Ephemeris Center: EARTH Extended: NO ------------------------------------------------------------------------------------------------------------------------------------ 17773( 3) - 10-Mar-2025 14:09:12 - [ 3] Visit: 01 Visit Requirements: VISIBILITY INTERVAL NO GYRO BIAS UPDATE ON MOVING TARGET On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 PLUTO STIS/CCD ACQ F28X50LP MIRROR ACQTYPE=POINT 1 8 S (8 S) SEQ 1-3 NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 2 CHARON STIS/CCD ACCUM 52X0.2 G430L 4300 WAVECAL=NO 1 1777 S (1777 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 WAVE STIS/CCD ACCUM 52X0.2 G430L 4300 1 DEF (0 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ PLUTO 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A CHARON 2 STIS/CCD ACCUM 52X0.2 G430L 4300 none none none 1 1 N/A CHARON 2 STIS/CCD ACCUM 52X0.2 G430L 4300 none none none 2 1 N/A WAVE 3 STIS/CCD ACCUM 52X0.2 G430L 4300 none none none none 1 N/A 17773( 3) - 10-Mar-2025 14:09:12 - [ 4] Summary Form for Proposal 17773 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 52X0.2, F28X50LP ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, ACQ ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT, WAVECAL=NO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements SEQ 1-3 NON-INT, VISIBILITY INTERVAL NO GYRO BIAS UPDATE ON MOVING TARGET ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G430L, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names CHARON, PLUTO, WAVE ------------------------------------------------------------------------------------------------------------------------------------